San Giovanni in Fonte is a richly decorated Late Antique baptistery, now part of the Cathedral complex of Naples. It is connected to the city’s old cathedral, Santa Restituta, traditionally attributed to Constantine the Great. The baptistery is generally attributed to Bishop Severus, who built it around 400, though it was perhaps restored later around the mid 5th century. 

The baptistery can be entered through a chapel at the end of its right aisle. A second entrance consists of a portico connecting it to the bishop’s palace. It has a shallow, wide white marble font at its center.  

Its dome is richly decorated with mosaics; its various scenes are set against a blue ground. At the center of the dome is a gold Christogram with an alpha and omega set against a starry sky. The Hand of God over it holds a wreath. The medallion is surrounded by a wide decorative band of vegetation, vases, and various birds - peacocks, pheasants, and a phoenix. Eight decorative bands radiated from the center, dividing the rest of the dome into eight sections. They are also decorated vegetation, vases, and birds. The upper register of each section has birds flanking vases, over which are blue and gold draperies. 

The lower registers depicted scenes from the New Testament, though only four have partially survived. One scene depicts the Wedding of Cana, where Christ miraculously turned water into wine. Interestingly, this also includes Christ meeting the Samaritan woman at the well. There is a depiction of the Traditio Legis  (Christ giving the Law) with Christ standing on a globe, handing the Law to St. Peter. Fragments of the Miraculous Catch of Fish have also survived, showing Christ telling Peter to cast his nets into the sea. Another fragment depicts the Women at the Empty Tomb, when an angel announced Christ had risen from the dead. 

The four squinches of the dome are decorated with a Winged Man, Winged Lion, and Winged Bull - the Eagle is now lost. These figures represent the Four Evangelists: Matthew, Mark, Luke, and John.  Above them are pairs of deer or lambs. The four deer drink from flowing water – which could be interpreted as the Four Rivers of Paradise. There is also a depiction of the Good Shepherd between one of the pairs of lambs. Between the Evangelists are full-length figures holding crowns, which perhaps represent Apostles.